In a conventional AC-to-AC converter (in particular converting a variable frequency AC voltage to a fixed frequency AC voltage) a first section may convert an AC-voltage (or current or power) to a DC-voltage (or current or power) generated at a so-called DC-link. A second section of the converter may convert the DC-voltage at the DC-link to a (in particular fixed frequency) AC-voltage (or current or power). In order to protect electronic components comprised in the AC-to-AC converter the conventional converter may comprise a so-called voltage clamp system connected between terminals of the DC-link. For example, a permanent magnet generator may release a current (in particular during shutdown) which may result in an overvoltage of the DC-link. For maintaining the voltage within the operational (switching) range of the semiconductor devices of the power converter system (also referred to as AC-to-AC-converter) the voltage clamp system comprises an isolated gate bipolar transistor (IGBT) or a similar force commutated device and a voltage clamp resistor (also known as a braking resistor or dynamic braking resistor). The overall combination of the control system managing the voltage clamping IGBT, the voltage clamping IGBT and the voltage clamping resistor may be referred to as the “voltage clamp”.
In a conventional power converter so-called DC-link capacitors are connected between the terminals of the DC-link. These capacitors are dimensioned such that the resulting (unclamped) overvoltage is maintained within the ultimate voltage limit for any and all of the power components connected to the DC-link of the power converter.
If the “voltage clamp” operates correctly, the dc link voltage is maintained at a level where switching of the other power semi-conductor devices connected to the dc link can continue. However, in an event of a non-operation of the voltage clamp, the DC-link capacitors have to act as the passive energy dump, in order to store the energy of the typically high inductance characteristics of the permanent magnet generator. To act as an effective energy dump, the capacitors require large values of capacitance. To achieve the large values of capacitance required to achieve the passive energy dump of sufficient size to absorb the energy released from the generator and to keep below the ultimate voltage limit for the power components comprised in the converter, generally electrolytic capacitors are selected. Electrolytic capacitors offer a very economic solution for low voltage solutions, say 800 V or 1100 V DC-link voltages, however at higher voltages metalized polypropylene capacitors are generally the preferred choice. Metalized polypropylene capacitors (MP capacitors) may offer a much higher ripple current rating than the equivalent electrolytic capacitors and may have also much longer lifetime. The disadvantage of MP capacitors however is that for the same capacitance value they are much larger and have a much higher cost. Thereby, the overall cost of the power converter may increase.
There may be a need for a high integrity voltage clamp system which can be used in an AC-to-AC power converter and which allows to construct the AC-to-AC power converter in a cost-effective way. Further, there may be a need for an AC-to-AC power converter which is cost effective and which has a compact dimension. Further, there may be a need for a voltage clamp effectively protecting electronic components comprised in the AC-to-AC power converter without increasing the costs of the power converter in an excessive manner. Further, there may be a need for an AC-to-AC power converter which may deal with the ultimate voltage limiting function without having to include capacitors of large capacitance values.